Getting the camshaft that is right for YOU

The most important thing to remember when designing your engine is
how it will be used most often. All of the components that you
choose for your engine must match your primary use (and each
other) or you will be disappointed with the results. This is especially
true for internal engine parts such as pistons, heads, and cams. If you
make a mistake choosing one of these items you will have a lot of
expense and work ahead of you to make the corrections.
Over-camming the engine is the most common mistake made when choosing a
cam. A big cam will not give better bottom end power and big horsepower
numbers also won't happen at low rpms without big cubic inches or a
supercharger.
Another common mistake is building a high compression engine and then
choosing a Torque/City or RV/Commuter camshaft to match the type of
driving the vehicle will be used for. Unfortunately this will produce
cylinder pressures that are too high for pump gas to handle. This is
the way to go only if you plan to run on propane (107 octane) or natural
gas (130 octane).

So how DO you choose your camshaft?

First, you must decide how the vehicle is going to be used most often.
If you are using it daily to go back and forth to work, how much of that
time is in city traffic? And how much of your traffic time is spent
sitting in line at the light? These questions might seem unnecessary,
but if you spend 25% of your time at idle and another 50% of your time
in traffic between 0 and 50 KMH (30 MPH), this information becomes
important. This would be the speed that determines the low end of your
RPM range.
Next you will have to determine your usual cruising speed and/or top
speed. If you have a tach it is easy to get your normal operating range
by checking your engine speed as you drive. If you don't have a tach in
your vehicle some calculations will be required to determine what RPM
you are running at. For this you will need to know your Tire Outside
Diameter (O.D.) and your Rear Axle Ratio. If you do not know your Rear
Axle Ratio there is often a tag on one of the differential cover bolts
that will tell you. Or you can find out the hard way:

Block the front wheels

If your vehicle doesn't have posi:

Raise one of your rear wheels off of the ground and properly support
your vehicle

Put a chalk mark on the raised tire and on the driveshaft

With the transmission in neutral and the emergency brake off rotate
the tire two full turns (to compensate for the action of the
differential) while counting the number of times the drive shaft
turns.

If your vehicle has posi:

Raise both of your rear wheels off of the ground and properly
support your vehicle

Put a chalk mark on the same position of both tires and on the
driveshaft

With the transmission in neutral and the emergency brake off rotate
both tires in the same direction one full turn while counting the number of times the drive
shaft turns.

Check to make sure the lines on the tires still match position. If
they aren't in the same positions as at the start something is
wrong.

The number of driveshaft turns is your Rear Axle Ratio. The common gear
ratios are 3.08, 3.23, 3.55, 3.73, 3.91, 4.10, 4.56, and 4.88.

To get your correct tire diameter measure from the ground to the center
of the wheel and multiply by 2. Measuring from any other point on the
tire will give you a larger diameter than your vehicle actually uses.

Keeping your idle requirements in mind, work out the low end of your RPM
range. For a vehicle that is driven daily in the city it will be your
idle rpm. For a drag vehicle it will be the launch rpm. For an oval
track vehicle it will be your rpm at re-start.
Now repeat the formula for your top speed. For a street vehicle the
highway cruising speed (90 KMH or 60 MPH) will be the top of your torque
range. For a drag vehicle peak horsepower and rpm will be at the fast
end of the strip. For an oval track vehicle peak horsepower and rpm
will be at the fast end of the straights.
Now you know what RPM range you require for your driving.
The optimum RPM range of a cam is really only about 3000 RPM from the
beginning of the torque range to the end, with another 1000 rpm to peak
horsepower. They will operate above and below this but not to the best
performance. If you require an RPM range that is wider than this you
will have to make a compromise. Which RPM range is the most important
to you? For hydraulic lifter cams, Rhoads variable lifters will add
about 1000 rpm to to lower end of the camshaft's normal operating range.

Basic Parts Selection Guide

This guide uses 8 typical performance levels from mild to wild. If you
are not sure which range to use, be conservative. If you choose
too high an rpm range it will be less reliable, harder on parts, and
rarely be used. An rpm range that is lower than you need will still be
used even if your top end is slightly limited.
Each performance level suggests the range of Accelerated Motion
camshaft, air/fuel requirements, compression ratio, exhaust type, gear
ratios, and ignition, that will work together the best for that
performance level. Maximum engine HP (not including nitrous or
supercharging) and expected idle speed are also shown.
The RPM range that a cam works best in will change with the
engine size and head design. Recomendations shown in this guide are for
average engines.
Please see the parts catalogue
for specific engine recomendations.

TORQUE AND CITY

ENGINE CAM RANGE*
151 4 cyl. up to 19477
181 4 cyl. up to 19767
189 6 cyl. up to 19393
250 6 cyl. up to 19686
300 6 cyl. up to 20220
302 V8 up to 19567
350 V8 up to 19827
400 V8 up to 19830
454 V8 up to 20190
500 V8 up to 20617
*The cam range refers to the 2nd half of the Accelerated Motion part number.
Compression: Gasoline 9.0:1 or less, Propane 10.5:1 or less
Exhaust: Stock or dual exhaust
Fuel Inj.: Should work with factory EFI computers
Gear Ratio: 3.7:1 and lower (numerically)
Idle: 600 rpm or less
Ignition: Recurved distributor and electronic ignition

PRO STREET AND RACE

(3000 rpm and up)
Super top-end performance. Manifold vacuum might be too low for power
brakes and automatic transmission modulators. NOT for daily driving.
Mild port work and multi-angle valve grind are recommended.
Automatic transmissions will require a high stall torque converter.

ENGINE CAM RANGE* AIR/FUEL RANGE HORSEPOWER
151 4 cyl. 23477 and up 369 cfm and up 181 HP and up
181 4 cyl. 23767 and up 443 cfm and up 217 HP and up
189 6 cyl. 23393 and up 462 cfm and up 227 HP and up
250 6 cyl. 23686 and up 611 cfm and up 300 HP and up
300 6 cyl. 24220 and up 734 cfm and up 360 HP and up
302 V8 23567 and up 738 cfm and up 362 HP and up
350 V8 23827 and up 856 cfm and up 420 HP and up
400 V8 22830 and up 978 cfm and up 480 HP and up
454 V8 24190 and up 1110 cfm and up 545 HP and up
500 V8 24617 and up 1223 cfm and up 600 HP and up
*The cam range refers to the 2nd half of the Accelerated Motion part number.
Compression: Gasoline 10.5:1 and up, Propane 12.0:1 and up
Exhaust: Dual exhaust with headers
Fuel Inj.: Race EFI system required
Gear Ratio: 3.9:1 and higher (numerically)
Idle: 850 rpm or more, will be rough
Ignition: Performance distributor and electronic ignition

What makes one cam in the range better than another?

There are many myths and legends about which cam is better, single
pattern (intake and exhaust the same) or dual pattern (intake and
exhaust different). The fact is that unless your exhaust ports are very
restricted there is no way to tell. On the dyno you would always be
comparing apples to oranges.

Lobe Center Separation is as big a consideration as duration.
Lobe center separation plays a role in determining how much valve
overlap (the amount of time the intake and exhaust valves are both open)
your engine will have and what your vacuum and idle quality will be.
Street cams with wide lobe center separation (114) generally will have a
good idle, high vacuum, and a nice wide RPM range. Separations closer
to 108 (less separation means more valve overlap) can create problems
for some computer engine controls due to their rougher idle and lower
manifold vacuum. They have a shorter RPM range but produce much
stronger mid-range power with some improvement to the top end.

Now we get to valve lift. Many customers believe that the cam
with the highest lift will perform much better than a moderate lift cam.
While it is true that a high lift cam will provide better flow by
getting the valve further out of the way, there are limits to this as
well as other considerations.
The high rate of lift required to achieve high lift on a short duration
cam is very hard on the valve train and causes valve to piston
interference problems (especially with narrow lobe separations). High
lift also causes several other problems including valve spring retainer
to valve guide/seal interference, rocker arm to stud interference, and
valve spring coil bind. Adjustable rocker arms are often required to
take up any clearance created in the valve train when the valve is
closed. All of this must be checked and corrected before a high lift
cam can operate properly.
If you are using a cam with enough duration to make a high lift
effective, the usual limit for noticable improvement is reached when the
lift equals 25% of the valve diameter. This means that if your valve
diameter is only 1.84 you will get the best flow at only .460 lift.
Lifts higher than 25% of the valve diameter will add more duration at
the maximum flow point but excessive lifts will cause more problems than
they are worth in an engine that is not fully race prepared.
The bottom line on cam lift is that it is pointless to go overboard.
All newer designs have adequate lift for the operating range of the cam.
The minimal gains from an extra high lift cam are not worth the extra
work in a street vehicle.
Here are some basics for choosing a cam from your range:

Large cars: Use short or medium duration, exhaust can be longer.
Wide lobe seperation is better.

Small Cars: Use long intake duration, exhaust can be longer.
Lobe center seperation can be short.

Propane and Natural Gas: Use short or medium duration single
pattern. Wide lobe seperation is required.

Trucks: Use short duration. Wide lobe seperation is better.

Nitrous or Supercharger: Use long duration. Wide lobe seperation
is required. Supercharger compression must be lower than shown in the
guide.

Turbos: Use short or medium duration, exhaust can be shorter.
Wide lobe seperation is required. Compression for turbos must be lower
than shown in the guide.

Accelerated MotionPerformance Products

Last updated August 2012.
(C) Copyright 1996-2012 by Doug Friesen,
doug@amotion.com. All rights reserved.
The Accelerated Motion name and logo is a registered trademark of
Doug Friesen. All other marks, names and part numbers are the
property of their respective owners and are used for the sole
purpose of promoting sales and proper use of their products. The
information contained at this site is accurate to the best of my
abilities and is subject to change without notice.